The present invention pertains to a heterostructure acoustic charge transport (HACT) devices and more particularly to an arrangement for improving device performance by reducing the magnitude of the surface acoustic wave (SAW) required for effective HACT device operation.
An HACT device employs a powerful ultra high frequency (UHF) SAW propagating on the top, highly polished surface of a wafer of piezoelectric semiconductor material, usually gallium arsenide (GaAs), to bunch mobile charge carriers in the extrema of the SAW electrical potential and to then transport these discrete charge packets at the speed of sound through the semiconductor material, as is described in detail in U.S. Pat. No. 4,893,161, entitled "Quantum-Well Acoustic Charge Transport Device," issued to William J. Tanski, Sears W. Merritt, and Robert N. Sacks. The SAW thus functions similarly to the clocking signal in a conventional charge-coupled device (CCD), but without the need for the complex interconnections which CCD's require.
The very weak piezoelectricity of GaAs (k.sup.2 =7.4.times.10.sup.-4 for the Rayleigh mode on {100}-cut, &lt;110&gt;-propagating GaAs) dictates that the great majority of the energy in the SAW is manifested as mechanical energy and only a small portion of the total energy is manifested through the electrical potential which accompanies the SAW. It is this electrical component of the total SAW energy which bunches the charge carriers to form distinct packets and which transports these packets, representing the input signal, through the HACT device. Accordingly, present day ACT and HACT devices require large (about one Watt) acoustic power levels in order to realize the voltage required (about one Volt) to effect coherent charge packet transport within the HACT channel, synchronous with the SAW clock signal.
Therefore, it is an object of the present invention to provide an HACT device which includes a greatly reduced acoustic power requirement for achieving coherent, synchronous charge transport.